Photovoltaic (PV) cells, capable of converting sunlight directly into electricity, could ultimately be the solution to global energy demand. When the first crystalline silicon (c-Si) PV device was demonstrated in 1954, the envisaged manufacturing costs made this technology impractical for large scale power generation. Thereafter, exploiting new materials and fabrication procedures has attracted a growing effort to realize lower-cost photovoltaic technologies. In particular, the recent emergence of organometal-halide perovskite based solar cells promises to deliver one of the lowest cost technologies that is capable of converting sun light to electricity at the highest efficiencies.
In perovskites, as well as for most ionic crystals, the coordination number for ions at the crystal surfaces is always lower than in the bulk material. The consequent non-stoichiometric material comprises a local excess of positive or negative ions, depending on the Miller index of the exposed surface. Such polar surfaces are known to be unstable, and their apparent natural occurrence has been associated with adsorption of foreign atoms, including oxygen and hydrogen from the moisture in air, which can passivate the crystal surface by balancing the local excess charge. For the particular case of a crystal of a perovskite such as CH3NH3PbX3 (where X is one or more halide anions), under-coordinated metal cations at the crystal surface would be unstable in air and can form lead oxide and hydroxide species when exposed to oxygen and water from the air. Similarly, under-coordinated organic cations at the crystal surface may bind with water molecules via hydrogen bridges, which causes hydration of the crystal surface. Conversely, halide anions in crystalline materials have been reported to be relatively stable to air exposure (Abate, A. et al., Journal of Fluorine Chemistry 130, 1171-1177 (2009)). Therefore, while it would be expected to find fewer “free” metal cations and organic cations when the devices are processed in air, it is likely that an excess of under-coordinated halide anions would be present at the crystal surface.
In U.S. Pat. No. 7,491,642 organic passivating layers have been investigated for silicon. The process described therein comprises carrying out chemical reactions of the silicon surface with organic passivating agents to produce passivating groups that are directly bonded to the surface by covalent (for instance Si—C) bonds.
As another example, as a means to control the growth of metal chalcogenide nanoparticles and ensure that they are well dispersed in a solvent of choice, coordinating ligands are employed (Ip A. et al., Hybrid passivated colloidal quantum dot solids, Nature Nanotechnology, 577, 7, (2012)). During the synthesis these ligands will typically be long alkane chain acid terminated molecules. These ligands make the nanocrystals highly soluble, advantageous for solution processing. However, since the ligands contain long insulating chains, they tend to prevent direct contact between the nanocrystals when processed into a thin film. This is overcome by performing a ligand exchange, whereby a solid as deposited nanocrystal film is rinsed in a solution of a different double ended short-chain ligand. Through mass-action, predominant ligand exchange occurs with the long chain ligands being replaced by the smaller double ended ligands. The short-chain double ended ligands enable closer proximity between the nanocrystals and can also bind to two different nanocrystals at the same time, “cross-linking” the nanocrystals, making the film insoluble. In contrast to metal chalcogenide nanocrystals, the metal halide perovskites described herein are typically fabricated directly as continuous layers rather than collections of nanoparticles, and often comprise large crystalline domains with domain sizes typically on the order of hundreds of nanometres to micrometers, much larger than the film thickness. Hence, there is no requirement to employ ligands in order to improve the interconnection between perovskite crystals, as is required for metal chalcogenide nanocrystals.
In comparison to c-Si and other thin-film semiconductors, defect sites and under-coordinated cations and anions in metal halide perovskite materials have not thus far been considered deleterious to device performance. However, the inventors have unexpectedly found that elimination or passivation of defect sites and surface states in the perovskite material allows improved device efficiencies. It is thus an object of the present invention to provide devices comprising passivated metal halide perovskites.